1. Field of the Invention
The present invention relates to an optical memory disc, upon which a laser beam is impinged so as to record, read out or erase information onto or from the optical memory disc.
2. Description of the Prior Art
Recently, an optical memory system has been developed, which optically stores information in high density and in mass storage order. In the optical memory system of the add-on-memory type or the erasable memory type, guide signals or guide addresses are normally recorded on the optical memory disc along circular guide grooves or along a spiral guide groove so as to control the optical beam position. In order to ensure an accurate operation, the concentricity of the guide grooves and a rotation center of the optical memory disc must be maintained at a high accuracy. If the rotation center of the optical memory disc is displaced from the center of the guide groove, the tracking operation becomes difficult due to whirling.